The invention pertains to a microwave pulse generator for generating microwave pulses in the nanosecond range according to the characteristics of the preamble of Claim 1.
Such microwave pulse generators are customarily employed for precise distance measurement in radar systems, particularly pulse radar systems.
Such microwave pulse generators are described in, for instance, DE 197 02 261 C2. The microwave pulse generators there have the advantage that the circuitry expense is considerably reduced in comparison to other microwave pulse generators by generating pulses of a suitable duration that are provided for the voltage supply of a microwave oscillator. Moreover, the known arrangement does not require any expensive charge-coupled diodes. The pulse in the nanosecond range, which determines the duration of the actual microwave pulse, can be generated by a simple pulse-shortening stage.
In a refinement, the coupling of pulse-shortening stage and oscillator can be improved by a driver stage and/or a decoupling stage. Should the pulse-shortening stage be constructed such that it generates an inverted output signal, this can be compensated by an inverting driver stage. The decoupling stage can advantageously be implemented by a circular stub.
The microwave pulse generator from DE 197 02 261 C2 will be explained further on the basis of three figures. In FIG. 1, a pulse generator, of which the output signal is fed to a pulse-shortening stage 2, is designated 1. Pulse-shortening stage 2 generates pulses in the nanosecond range, which are fed to the input of a driver stage 3. The signal amplified by driver stage 3 is fed to a decoupling network 4, which is connected at its output end to the supply input of a microwave oscillator 5. The output signal of microwave oscillator 5 can be picked off at an output terminal 24.
The pulse generator supplies a pulse train with a predetermined pulse period. According to FIG. 2, downstream pulse-shortening stage 2 can have an input terminal 6, which is connected via a resistor 7 to the base of an npn transistor 11 and via a resistor 8 to the base of an npn transistor 10. A capacitor 9 between resistor 8 and the base of transistor 10 is connected to ground. The collector of transistor 10 is connected to the base of transistor 11, and the emitter of transistor 10 is connected to ground. The emitter of transistor 11 is likewise connected to ground. The collector of transistor 11 forms the output circuit of the pulse-shortening stage and is coupled to the input circuit of downstream driver stage 3. For this purpose, the collector is connected to supply voltage terminal 17 via series circuit consisting of three resistors 12, 13 and 16. The center tap of the series circuit of resistors 12 and 13 is connected to the base of pnp transistor 18 and the center tap of the series circuit of resistors 13 and 16 is connected to the emitter of transistor 18. Its collector is connected to an output terminal 19. Resistor 16 is connected to ground at both ends via decoupling capacitors 15 and 16.
According to FIG. 3, the microwave oscillator has a supply terminal 20. The latter is connected via a resistor 21 to a circular stub A and a xcex/4 line B. At its output end, xcex/4 line B is wired to output terminal 24 via a capacitor 23 and is connected to ground via the load path of a field-effect transistor 25 and a resistor 26 connected in series thereto. The gate terminal of field-effect transistor 25 is connected to ground via an inductor 27.
Output 19 of driver stage 3 is connected to supply terminal 20. A pulse train with a predetermined period is supplied to input terminal 6. The incoming pulse from pulse generator 1 is shortened in pulse-shortening stage 2 to length tp. This is done in the embodiment of FIG. 2 by virtue of the fact that the positive edge of the incoming pulse switches transistor 11 into the conductive state upon exceeding its base-emitter potential. Thereby, voltage divider 12, 13, 16 is powered and thus sufficient voltage is dropped across resistor 13 to switch transistor 18 into the conductive state. At the same time, the positive edge of the incoming pulse is delayed via RC element 8, 9 by the time defined by it. By selecting the fast transistor appropriately, this delay time can be adjusted from fractions of a nanosecond to the length of the incoming pulse. After this delay time has elapsed, transistor 10 is switched to become conductive, so that the voltage at the base of transistor 11 is reduced to the saturation potential of transistor 10. Transistor 11 thus returns to the high-ohmic state and thereby also blocks transistor 18. Accordingly, a short pulse of length tp is available at output 19 and can moreover be loaded low-ohmically. Network 14, 15, 16 serves only to block the operating voltage that is applied to terminal 17. Pulse-shortening stage 2 and driver stage 3 complement one another in the present example by each inverting the signal to be processed, whereby a noninverted signal can be picked off at output 19.
The signal thus obtained, with a pulse duration corresponding to the duration of the microwave pulse, is furnished to the microwave oscillator via terminals 19 and 20. The microwave oscillator consists of a transistor 25, embodied in the present example as a gallium arsenide field-effect transistor. A suitable bipolar transistor could also be used, however. Furthermore, the inductor is embodied as an inductive TEM line segment. The resonant circuit of oscillator 5 is composed of this line segment 27 and the internal transistor capacitance between gate and drain for FETs, or base and collector for a bipolar transistor. Together with the transistor capacitance, line segment 27 constitutes a series resonant circuit that can be tuned via the length of line 27. The phase condition for the start of oscillation is additionally fulfilled by this. Resistor 26 is required to reduce the Q of the resonant circuit so that a rapid starting of oscillation is guaranteed. A resistor 21 is inserted in the feed line between driver stage 3 and decoupling network 4 to limit the current through transistor 25. Capacitor 23 serves to block the supply voltage and thus decouples the output signal of the oscillator.
Microwave oscillator 5 is designed such that it generates a CW signal upon application of a supply voltage to terminal 20 at the resonant frequency of the determining resonant circuit. In matching the line length of line segment 27, care should be taken to consider the transformed component of the self-inductance of resistor 26 parallel to inductor 27.
As already described, the supply of voltage to the microwave oscillator is accomplished by a pulse of length tp. In order to decouple the pulse-shortening stage and the downstream driver stage 3, it is fed via the decoupling network consisting of a circular stub A and xcex/4 line B to microwave oscillator 5.
To achieve a rapid oscillation onset and decay behavior, the source terminal must be connected to ground via a resistor 26. This resistor 26 reduces the Q of the resonant circuit sufficiently that the oscillator has achieved its maximum amplitude after half the pulse length, that is, precisely at the maximum of the pulse amplitude. From there on, the pulse amplitude, and thus also the amplitude of the microwave oscillation, decreases until the pulse amplitude has again reached zero.
The coherence of the microwave oscillation is achieved because the pulse supplying oscillator 5, roughly a nanosecond in duration, has a small rise time in the vicinity of 250 ps and thus already couples a spectral energy component at the resonant frequency into the oscillator. Thus the initial phase of the microwave signal is firmly shaped.
The microwave pulse is decoupled at output 24 of oscillator 5 via capacitor 23. Here, however, the shortened pulse is superimposed on the microwave pulse, but can be removed via a highpass filter. If the microwave pulse that has been created is relayed in a waveguide, however, it is possible to dispense with the additional highpass filter, since the waveguide has the same behavior.
The above-described arrangement permits the shortening of the pulses fed in from pulse generator 1 as adjustable pulse lengths from roughly 0.5 ns up to the length of the input pulses. The adjustment of the pulse length is done by way of ohmic resistor 8 and capacitor 9. The ohmic resistor is embodied here as a trimming potentiometer. Capacitor 9 has a fixed capacitance. The adjustment of trimming potentiometer 8 has thus far been done generally by hand.
Due to the very high frequency components in the operation of the microwave pulse generator, however, the employment of trimming potentiometers is critical. The high-frequency components are caused by the very short rise times of the input signals. Due to the mechanical structure of the arrangement, the parasitic capacitances and inductances of the circuit arrangement are relatively large. The high-frequency components are also scattered to a strong extent. In sum, the entire circuit arrangement thereby becomes particularly sensitive with respect to approach to or touching of the potentiometer.
It has additionally been shown that the manual manipulation of the potentiometer for setting the pulse length when tuning is not optimal in the manufacturing of microwave pulse generators. The hand-tuning of the trimming potentiometer hampers automated manufacturing.
Here is where the present invention takes its start.
The objective of the present invention is viewed as the modification of the previously known circuit arrangement such that, on the one hand, the entire circuit becomes less sensitive to someone approaching and, on the other, there can be automatic tuning, which avoids manual handling of the circuit arrangement.
The problem is solved by a microwave pulse generator with the characteristics of Claim 1.
Refinements are the object of the subordinate claims.
The invention is based essentially on replacing the previously used RC element having a potentiometer with a device that adjusts the amplitude and/or length of the microwave pulses to be generated according to an electronic control signal.
According to a preferred embodiment of the invention, such a device is expediently realized by a fixed resistor and a varicap diode, which changes its capacitance via a voltage applied to the diode.
The essential point in the present invention is that the setting of the microwave pulses is done without a potentiometer.
The device for controlling the amplitude and/or length of the microwave pulses to be generated expediently has a capacitor for DC decoupling of the electronic control signal.
The microwave pulse generator according to the invention can be a component of a sensor device, in which a microprocessor provides an electronic control signal for the pulse-shortening stage by way of an adjustable voltage regulator. The pulse-shortening stage and/or the microwave oscillator is also connected to the microprocessor via a feedback device. The feedback device can be, for instance, a test bench.